Drilling fluids used in the drilling of subterranean oil and gas wells along with other drilling fluid applications and drilling procedures are well known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply “muds”. The drilling fluid should carry cuttings and other particulates from beneath the bit, transport them through the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs. It will be appreciated that within the scope of the claimed invention herein, the term “drilling fluid” also encompasses “drill-in fluids” and “completion fluids”.
Drilling fluids are typically classified according to their base fluid. In water-based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water. Nonetheless, the water is the continuous phase. Brine-based drilling fluids, of course are a water-based mud (WBM) in which the aqueous component is brine. Oil-based muds (OBM) are the opposite or inverse. Solid particles are suspended in oil, and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds can be either all-oil based or water-in-oil macroemulsions, which are also called invert emulsions. In oil-based mud the oil can consist of any oil that may include, but is not limited to, diesel, mineral oil, esters, or alpha-olefins. If the oil is synthesized as compared to being produced by conventional refining techniques, the mud is a synthetic-based mud or SBM. The non-aqueous muds may be referred to as a class by the short-hand O/SBM.
It is apparent to those selecting or using a drilling fluid for oil and/or gas exploration that an essential component of a selected fluid is that it be properly balanced to achieve the necessary characteristics for the specific end application. Because drilling fluids are called upon to perform a number of tasks simultaneously, this desirable balance is not always easy to achieve.
A spacer fluid is a liquid used to physically separate one special-purpose liquid from another. Special-purpose liquids are typically subject to contamination, so a spacer fluid compatible with each is used between the two. A common spacer fluid is simply water, but solvent-based spacers and spacers that contain a majority of mutual solvents are also typical. However, other chemicals are often added to enhance the fluid's performance for the particular operation. Spacers are used primarily when changing mud types or changing from mud to a completion fluid. In the former, an oil-based fluid must be kept separate from a water-based fluid. Another common use of spacers is to separate mud from cement during cementing operations. For cementing, a chemically treated aqueous spacer or sequence of spacers usually separates drilling mud from the cement slurry subsequently pumped downhole. Cleaning spacers are also extensively used to clean the casing, riser and other equipment after drilling a section of wellbore. Cleaning spacers not only remove the remaining drilling fluid from the wellbore, but also cuttings, weighting agent particles, e.g. barite, and other residual oily debris and contaminants.
Indeed, displacement of the drilling or drill-in fluid to become clear brine in the wellbore has become a very important process in the overall well bore completions program leading to optimized hydrocarbon recovery. Poor displacement can lead to less than planned production rates due to formation damage due to mud residue deposits left on wellbore surfaces, including metal and/or formation faces. Other consequences of an inefficient displacement include stuck packers, completion tool setting problems, increased rig and filtration time, increased disposal costs, and increased corrosion pitting. Displacement fluids and spacers typically contain at least one surfactant, optionally at least one viscosifier, and are designed to be as close to “plug flow” or laminar flow as possible. On the other hand, some spacers are designed to provide turbulent flow to further scour away oil and oily debris. They may be used to displace both O/SBM and WBM.
It would be desirable if compositions and methods could be devised to aid and improve the ability to clean up residual OBM, SBM, and any debris or other contaminants, and to remove them more completely, without causing additional formation damage.